Nickel-aluminum alloy steel for production of gears, steel plate and the like



United States Patent M 3,284,191 NlCKEL-ALUMINUM ALLEY STEEL FUR PRODUC-TIUN 0F GEARS, STEEL PLATE AND THE LIKE Peter Paul Hydrean, Mahwah,Damian Vincent Gullotti, Jersey City, and John Trirnhle Eash, Allendale,N.J., and Ralph Bertram Grenville Yeo, Pittsburgh, Pa., assignors to TheInternational Nickel Company, lino, New

York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 9, 1966,Ser. No. 526,034 13 Claims. (Cl. 75l124) This is a continuation-in-partof application Serial No. 306,340, filed September 3, 1963, nowabandoned.

The present invention relates to low alloy, precipitation-hardenablesteels, and more particularly to low alloy, nickel-aluminum tempagingsteels (i.e., steels which undergo a tempering reaction and,simultaneously therewith, a precipitation-hardening reaction upon heattreatment) of special composition which manifest an improved combinationof properties, including strength, hardness and toughness, of suchmagnitude that the steels are eminently suitable for diversified use,such as plates, gears, dies, etc.

Within the past decade, substantial improvements have been made withregard to the low alloy, precipitationhardenable steels. In particular,the nickel-aluminum class of such steels, to wit, those referred to asthe Ni-2% Al steels, have been advanced as particularly attractive forthe production of gears and dies as a result of their capability ofbeing produced within close dimensional tolerance upon hardening. Thatis to say, such steels, in contrast to many of the low alloy quenchedand tempered steels, harden with minimum distortion or warping and thisattribute is notably beneficial in processing the steels to gears andthe like. Some of the salient virtues of such steels are succinctly setforth by J. B. Seabrook in an article entitled Properties of Ni-AlAge-Hardening Steel, Metal Progress, vol. 79 (1961), pages 80 through 84.The author summarizes the 5% Ni-2% Al steel as providing moderatematerial cost, low processing cost, good forgeability and machinability,low distortion, strengths similar to conventional steels and agehardnesses up to Rockwell C53 and confirms that such steels possess goodmachinability at relatively high intermediate hardness levels, i.e.,such steels can be readily finish-machined prior to final hardeningwhich, of course, eliminates or minimizes the considerable difficultiesencountered in machining steels in the fully hardened condition.

However, a serious drawback characteristic of the foregoing 5% Ni-2% Alsteels has been their lack of high toughness at desired strength andhardness levels. What is presently needed is a low alloy steel of thenickelaluminum type capable of providing characteristics comparable tothose aforementioned including hardnesses of about Re 40 or higher,yield strengths of up to 200,000 pounds per square inch (p.s.i.), e.g.,150,000 to 200,000 p.s.i., etc., but with impact energy levels of atleast 20 foot-pounds (ft-lbs), e.g., 25 ft.-lbs. and higher, at roomtemperature as determined by standard Charpy V-Notch procedures. Thisdesideratum has presented a formidable barrier to commercial developmentand application.

The 5% Ni-2% Al steels disclosed in US. Patent No. 2,708,159 indeedrepresent a significant improvement over what had been accomplishedtheretofore. In accordance therewith, it was considered that the amountof carbon had to be correlated to the nickel and aluminum contentsexpressed by the stoichiometric relation NiAl. However, the steelsdisclosed therein had an average Charpy Keyhole Notch impact value onlyon the order of about 9 or 10 ft.-lbs. on bar specimens. As indicated inthat patent, it is necessary for such steels to possess 3,284,191Patented Nov. 8, 1966 a hardness of about 40 or above as measured on theRockwell C (Re) scale to qualify as good gear steels. Indeed, suchhardnesses were attained, but not with a high level of toughness. Thenickel-aluminum steels advanced in US. Patent No. 2,715,576 (theillustrative steels therein having nickel and aluminum contents of about3.6% and about 1.2%, respectively, i.e., a ratio of nickel to aluminumof about 3 to 1) are also afflicted with the same disadvantage, i.e.,low toughness. In this latter patent it was considered that a specialheat treatment prior to aging together with the utilization of a strongcarbide former, to wit, vanadium and/or colurnbium and/ or titanium, wasnecessary, the amount of the carbide former being related to the carboncontent. We have found that the approaches to the problem as set forthin the aforementioned patents markedly diifer from the concepts of thepresent invention. Perhaps it can be said that the nickel-aluminumsteels proposed heretofore have been preordained with a lack of goodtoughness at high strength levels, a fact seemingly confirmed by thedata presented in the above-mentioned article by J. B. Seabrook.

A further notable disadvantage of the aforementioned prior art 5% Ni-2%Al steels is their lack of versatility for diverse application. That isto say, such steels have been somewhat restricted to considerationsinvolving nitrided parts, e.g., gears. Seemingly, this aspect evolvesfrom the insufficient toughness which is characteristic of such steels,a lack of toughness which would, as a practical matter, commerciallyminimize (it not preclude) their use as a structural alloy steel in theform of, say, plate. Since steel plate is a significant tonnage item inindustry as a constructional material (including forms derived fromplate), it would be thus most advantageous to also provide an alloysteel capable of manifesting prop erties which would enable it to beused in the form of steel plate and the like and thus lend versatilityand diverse use to its role of application.

The problem of providing high toughness together with other desiredproperties in steels of the type under consideration is one ofparticular severity for a variety of rather complicated and interrelatedfactors. For example, in the type of alloys in question, there is abasic hardening reaction occurring in the formation of a precipitatingphase at the age-hardening temperature. Concurrently and in competitiontherewith, there is a tempering (softening) reaction. In other words,this overall combined reaction, referred to herein as tempaging,involves two competing forces which, to a degree, tend to oppose theeffect of each other. In addition, the picture is further complicated bythe nature of the initial transformation product formed during coolingfrom the austenitizing temperature and the effects of the hardeningreactions on the properties of the transformed matrix. Together withthese factors is the problem attendant the formation of complex carbidesand the combined influences of the elements in solid solution.

In approaching this complex problem, it was thought that by varying heattreatment conditions (particularly the rate of cooling) with regard tonickel-aluminum steels heretofore proposed, a practical solution mightbe achieved. It was found that by furnace cooling at a rate of about 300F. per hour from a solution treating temperature of about 1275 F.,Charpy V-Notch impact energies of only about 10 ft.-lbs. could beobtained together with Rockwell hardnesses of Re 43. Furnace cooling ata rate of F. per hour from the same solution treatment temperatureresulted in an increase in impact energy to about 16 or 17 ft.-lbs.;however, yield and tensile strengths were adversely affected. Since oneof the virtues of the type of steel under consideration is itsinsensitivity to distortion or warping upon age hardening, this approach(utilization of a slow furnace cool) was considered inappropriate from acommercial and economic viewpoint particularly since it did not appearthat sufliciently high levels of impact inergy could be attained. Inaddition, slow cooling rates merely served to prolong the period of heattreatment and this is also unattractive from the commercial viewpoint.

Although attempts were made to overcome the foregoing difficulties andother difiiculties, none, as far as We are aware, was entirelysuccessful when carried into practice commercially on an industrialscale.

It has now been discovered that low alloy, nickel-aluminun, tempagingsteels containing special and correlated amounts of carbon, nickel,aluminum, molybdenum, chromium, silicon and manganese can be providedwhereby toughness levels are achieved of a magnitude substantiallygreater than that heretofore attained with nickelaluminum steelsheretofore proposed, i.e., those set forth in the patents referred toabove. This marked improvement can be obtained without a concomitantdetrimental loss of strength and hardness. Perhaps it is worthy tomention at this point that the presence of the element tin should beavoided or kept to very low levels as will be further detailed since ithas been discovered that tin is a potent performer in adverselyaffecting toughness. Further, we have found that by overaging steelswithin the invention, a highly satisfactory and most expedient method isprovided for achieving desired combinations of properties, particularlyin steel plate of substantial section size. In addition, alloy steelscontemplated herein can be air cooled (as distinct from liquidquenching) prior to aging and a satisfactory combination of propertiescan be achieved.

It is an object of the present invention to provide new and improved lowalloy, nickel-aluminum tempaging steels.

Another object of the invention is to provide low alloy, nickel-aluminumsteels which undergo a tempaging reaction during heat treatment andwhich are characterized by a combination of impact energy, hardness andstrength manifestly superior to that heretofore characteristic of whatmight be considered as comparable steels.

The invention also contemplates providing low alloy, nickel-aluminum,tempaging steels capable of exhibiting Charpy V-Notch impact energies ofup to at least 50 ft.- lbs. at hardnesses of up to about Re 43 togetherwith yield strengths (0.2% offset) of up to approximately 200,000p.s.i., e.g., 150,000 p.s.i. to 195,000 p.s.i.

It is also an object of the invention to provide a special heat treatingprocess for accomplishing the foregoing.

It is a further object of the invention to provide low alloy,nickel-aluminum tempaging steels characterized by properties whichrender them eminently suitable in the form of steel plate as well asnitrided parts such as gears and the like.

Other objects and advantages will become apparent from the followingdescription of the invention.

In accordance with the invention, an optimum combination of propertiescan be achieved with tempaging steels having compositions (based onweight percentage) within the following most advantageous ranges: 0.04%to about 0.16% carbon, about 5%, and preferably about 6%, to 10% nickel,at least 0.7%, and preferably at least 0.8%, and up to about 1.1%aluminum, about 0.75% to about 1.5% molybdenum, up to 1% chromium,manganese in an amount up to not more than about 0.3%, silicon in anamount up to not more than about 0.3% and the balance essentially iron.Within these ranges high levels of resistance to impact (toughness) ofat least ft.-lbs., e.g., ft.-lbs., and up to 50 ft.-lbs. or higher atroom temperature as measured in accordance with standard Charpy V-Notchprocedures can be achieved together with yield strengths of 150,000p.s.i. (0.2% offset) and upwards of 200,000 p.s.i. in combination withhardness levels of about Re 37 to Re 43 and higher. In addition, thesteels manifest elongation values (measured at a gage length of one inchon 0.252-inch diameter bars) of well over 10%, e.g., 15% to 20%, andreduction of areas of at least 40%. A particular virtue of the steels isthat they can be satisfactorily provided in the form of plate as well asfor manufacture into gears and dies. In this connection, it is ratherunusual to find a steel which is capable of providing the degree ofhardness necessary for a gear steel and still manifest a toughnesssatisfactory for a plate steel. Eminently satisfactory properties can beobtained in steel plate up to thicknesses of 2 inches or more.

A further benefit of the steels in accordance with the invention is thatspecial control of sulfur and phosphorus contents is not necessary inachieving a satisfactory level of properties. However, while up to 0.03%of each of these constituents can be present, it has been found thatwhere optimum toughness is desired (as determined by the capability ofthe steels to absorb impact energy) the respective amounts of phosphorusand sulfur should not exceed about 0.01%. This applies to'the steels inthe form of plate as Well as bar, rod, etc. Although the probability israther remote, tin which might possibly be picked up if scrap materialis used should not exceed 0.005% and most advantageously should be lessthan 0.003% since this element is a potent subversive to the attainmentof desired properties in the steels of the invention, particularly inregard to toughness.

In carrying the invention into practice, it is preferred in achieving anoptimum combination of properties that the steels be austenitized withina temperature range of about 1550 F. to 1700 F. and then liquidquenched. If austenitized much above 1700 F., excessive austentite graingrowth occurs with attendant loss in toughness. On the other hand,austenitizing temperatures of, say, 1525 F. can result in incompletetransformation to austenite. It is most advantageous to employ anaustenitizing temperature range of 1575 F. to 1650 F. Subsequent toquenching, the steels are tempaged at a temperature of 950 F. to about1050 F. A tempaging period of up to 30 hours or more can be used, but itis advantageous to avoid prolonged tempaging periods and 2 to 10 hoursis quite satisfactory. As indicated above herein, there simultaneouslyoccurs a tempering action which induces a softening response and aprecipitation action which induces an opposed hardening reaction. Thus,at tempaging temperatures of about 900 F. or below, e.g., 800 F., aslight tempering effect occurs but there is an insuflicient agingresponse. At temperatures of about 1050" F. and above, e.g., 1100 F., agreater degree of tempering coupled with overaging occurs and thisprovides a softening effect. However, tempaging temperatures at about1050 F. and over can be employed to advantage particularly with regardto alloy steels wherein the nickel content is at the high side of thenickel range, e.g., 6.5% or 7% to 9.5% or 10% nickel. Nickel has beenfound to be a potent hardener and confers a high degree of hardenabilityto the steels of the present invention. This high degree ofhardenability is particularly beneficial for produccing steel plate ofsubstantial thickness (section size). By overaging, to wit, employing atempaging temperature of 1050 F. or higher, e.g., about 1080 F. or 1100F. and up to about 1125 F., the hardness (and strength) of the steelswill be reduced (at 1050 F. the loss of strength is very slight), butthe toughness (impact energy levels) thereof will be appreciablyincreased. Significantly lowering the nickel content below 6.5% or 7%and thus reducing the hardness in an effort to obtain higher toughnessis inappropriate because of the attendant loss in hardenability.Further, by lowering the nickel content it does not at all necessarilyfollow that the toughness will be enhanced. Accordingly, where intendedapplications may not require the absolute maximum in hardness andstrength but would require the optimum hardenability and toughness,overaging provides a simple and expedient method for accomplishing suchan objective. However, the overaging temperature should not exceed 1125F. because of the danger of incurring retention of 5 excessiveaustenite. Thus, it is advantageous that the overaging temperature notexceed about 1100 F. Following the tempaging treatment the steels arecooled.

With further regard to the foregoing description of heat treatment, ifnecessary for a particular application,

e.g., where the configuration of the product to be produced is notconductive to quenching, the steels can be air-cooled from theaustenitizing temperature as opposed to liquid quenched. This isparticularly applicable with regard to the alloy steels having highernickel contents,

Le, 7% to 10% and preferably 7.5% or 8% to 10% nickel, and is especiallyadvantageous with regard to steels to be produced in the form of plate.As is known, many of the conventional constructional steels have a yieldstrength ranging from 30,000 psi. upwards to the order cooling has anadditional advantage in that distortion and 30 warping are furtherminimized.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/ or a better ap- In accordance Ingeneral, the steels were prepared in an induction furnace and processedinto ingot form. The ingots were soaked for a minimum of about 2 hoursat 2300 F. which was subsequently lowered to 2150" F. Thereafter theywere forged to l-inch by 3 /2-inch plate. After forging, the plates werereheated for about /1 hour at 2150 F. and then hot rolled in one pass toa thickness of about inch. The plates were cooled in air to 1800 F. andgiven a final hot-rolling pass. The resulting plates had a thickness of/2 inch and a width of about 4 inches and were cut into 7-inch lengths.Some ingots were reduced to %-.inch round bar by forging at 2150 F. to2- inch square bar and hot rolled to 4-inch round bar after heating to2150 F. subsequent to forging. The round bars (Alloys Nos. 1 to 7 ofTable I) 'and. plates (Alloys Nos. 8 to 16 of Table I) were machined to0.252-inch diameter specimens and subsequently subjected to test. Theheat treatment consisted of austenitizing at about 1600 F. for about 1hour, water quenching, tempaging at about 1000 F. for about 2 hours andair cooling. The properties obtained are set forth in Table 11 whereinthe 0.2% offset yield strength (Y.S.) and ultimate tensile strength(U.T.S.) are given in thousands of pounds per square inch (K.S.I.), theaverage Charpy V-Notch (C.V.N.) values obtained using standardprocedures (longitudinal direction) are given in foot-pounds (ft-lbs).the elongation values are given in percent (Elong. percent) andrepresent the measurements taken using a gage length of 1 inch, thereduction of area is given in percent reduction in area (RA. percent)and the Rockwell hardness as measured on the C-scale (Re) is given forthe values obtained both prior to and after tempaging (Re 501., Rc aged,respectively).

TABLE 11 Alloy No. Y.S U.T.S., C.V.N Elong., R.A., Re Re k.s.1 ksi.ft.-lbs Percent Percent Sol. Aged 195 203 50 1G 65 39 43 185 191 54 17no 41 166 172 98 21 71 38 38 178 185 32 18 69 39 41 192 199 27 17 G7 424 1 187 194 25 16 70 36 44 159 168 69 18 67 38 152 160 71 19 68 37 173186 47 17 64 41 1% 205 30 14 59 34 193 202 27 15 62 33 45 200 212 28 1645 178 184 35 15 63 42 203 33. 5 16 65 39 43 185 191 35 17 (16 39 41 166172 62 21 71 38 38 preciation of the advantages of the invention, thefollowing illustrative data are given:

A series of steels were prepared having compositions as 55 given inTable I.

i TABLE I Table II illustrates that yield strengths of up to about200,000 p.s.i. together with impact strengths of up to 50 ft.-lbs. (andeven much higher) can be attained in accordance with the invention. Theresults obtained from Al, Mo, Cr, Mn, Pcr- Per- Percent cent cent Ni,Percent Alloy N 0.

cent

1 Balance essentially iron with sulfur not exceeding about 0.004% andphosphorus not exceeding about 0.003%.

both the round bar and plate material were very satisfactory. Acomparison of round bar and plate results is afforded by Alloys Nos. 1and 14, 2 and 15 and 3 and 16, which pairs of alloy steels are of thesame composition.

TABLE III Alloy No. 0, Ni, A1, Mo, Cr, Mn, Si, Y.S., C.V.N.,

percent percent percent percent percent percent percent K.S.I. 1t.-ll s.

As indicated hereinbefore, it is rather diflicult and perhaps hazardousto assess the attributes of each of the individual elements; however,nickel exerts a pronounced hardenability and strengthening efiectwithout a concomitant deleterious loss in toughness. Lowering the nickelcontent adversely affects strength without an appreciable increase intoughness. The nickel content should not be less than 4.5% and theamounts of nickel and aluminum must be correlated such that the amountof nickel is at least five times the aluminum content if a 15 Sincesteels within the invention and which contain about 7.5 nickel or moreare relatively immune to fluctuation in carbon content as above-noted,it is thus most advantageous to employ low carbon contents, e.g., 0.04%to 0.1%, to obtain the optimal regarding weldability characteristics. Asis known, an attendant characteristic of high carbon contents is theincreased difficulty in achieving good weldability. Accordingly, lowcarbon contents are quite beneficial. However, carbon does contribute tohardenability and strength and this is most pronounced good combinationof properties is to be achieved. The t ni k l l v ls below Thus, theCarbon Content data in Table 11 further illustrate that if a hardnesslevel hould not be lower than 0.04% If a SatlSfaCtOIY of Re or higher isnecessary, a nickel content of over Pmatlon of P p 15 to be Obtfllned-Broadly p 5%, e.g., 6% or higher, is most advantageous, parti mg, thecarbon content should not exceed 0.25%; otherlarly if amounts of theother elements, notably aluminum 3O W156 can E? f f y affected- FOrOptimum and molybdenum, at the lower end of their ranges armachinabilrty and fa rlcabllity, the carbon content should not exceedabout 0.2%, and, most advantageously, should employed d h h not exceed016% a e wit t e invenion The role carbon i gg 61a Siml behavior as Therespective amounts of aluminum, molybdenum, fioes not at a necessan i Sb t 1 n manganese and silicon have also been found to be im- 1 Xample,the queue e an tempere car on s ee ortant in accordance with theinvention. Thus, the alu- That is to say, 1t does not necessarily followthat by uti minum content Should not exceed 12% and While 12mg lowcarbon to, say, (108%, amounts of aluminum as low as about 0.6% can bean increase 1n toughness Wlll. be achieved. In fact, and employed, it ismost advantageous that a range of 0.7% in accordance with the invention,at nickel contents above 40 o 0.8% t 1 1% l i b d, Si the steels about7%, e.g., 7.5% to 10%, the steels of the invention of the presentinvention are tempaging steels, if the when liquid quenched fromaustenitizing temperatures, amount of aluminum is appreciably lower than0.6%, are virtually insensitive, as a practical matter, to carbon e.g.,0.3% or 0.4%, the tempering effect can greatly over TABLE IV Alloy No.0, Ni, A1, Mo, Cr, Mn, Y. s., C.V.N.,

percent percent percent percent percent percent percent K.S.I. it.-lbs.

Aluminum Effect Chromium Efiect Manganese Efiect Silicon Eflect 1 Whilemanganese lowers toughness as the amount is increased, the general levelof toughness for Alloys Nos. 31, 32 and 33 was generally low becausethose Alloys contained 0.012%, 0.012%, and 0.011% of tin, respectively.

shadow the hardening effect and this brings about a rather severe lossin strength. On the other hand, significant 10 (longitudinal direction)were determined at room temperature.

TABLE V Alloy No. 0, Ni, Al. M0, Cr, Mn, S1, Y. S., C.V.N.,

percent percent percent percent percent percent percent K.S.I. ft.lls.

Mn and Si were below 0.2% and 0.25% respectively. Balance iron andimpurities.

amounts of aluminum above 1.2%, e.g., 1.5% and higher,

In addition to the data in Table V, it might be menare afflicted withquite a consequential eifect on toughness. troned that With a platethickness of the order of, say, Molybdenum is essential in obtainingnecessary strength /2 inch, the influence exerted by phosphorus andsulfur levels and to resist temper embrittlement but toughness is isgenerally more pronounced with regard to toughness. impaired if theamount of molybdenum is excessive. This is deemed attributable to thefaster cooling rate This is particularly significant when the molybdenumobtained in respect of a K2 inch plate as opposed to that contentexceeds about 1.5%. An increase in molybmanifested in respect of a plate4 inches thick. A subdenum above 1.5 other factors being equal, does notstantially complete martensitic structure results with the result in anappreciable increase in strength, but toughfaster cooling rate whereaswith a section size of 4 inches ness is rather strongly atfected. Whilethe molybdenum an appreciable amount of bainite is also present. Asbecontent can be present in an amount up to 2%, an optitween the twostructures, the former is thought to exhibit mum combination ofproperties is achieved if it is maina greater susceptibility to loss oftoughness. tained at a level of not more than about 1.5 In addi- It hasbeen previously pointed out above herein that tion, the amount ofmolybdenum should not be lower where desirable, overaging can beemployed to achieve than 0.5%; otherwise, the strength of the steels issigenhanced toughness particularly with regard to steels of nificantlyimpaired. A molybdenum range of 0.75% or the invention having nickelcontents of 7% to 10.0%. 0.8% to 1.5% has been found to be the mosteffective. This is illustrated by the data in Table VI. In this con- Theamounts of manganese and silicon should not exnection, the steels (platestock) were austenitized for 1 ceed 0.5% and preferably should notexceed about 0.3%. hour, liquid quenched and Fagfid at 1000 f 2 ur Highamounts of these elements have been found to have (H at Tr atm HTfA) oraged at 1050 F. for 2 an adverse effect on toughness. hours (HeatTreatment, H.T.B) or aged at 1100 F.

Where high hardenability is desired, it is advantageous for 2 hours (HeaTreatment,

TABLE VI Alloy 0. Per- Ni. Per- Al, Per- M0, Per- Cr, Pen Mn. Per- Si,Per- FLT. Y.S., C.V.N N0. cent cent cent cent cent cent cent K.S.I.ft.-lbs.

0.00 7.05 0.81 0. 93 0.10 0.19 0.17 A 173 47 B 105 52 41 0. 04 7. a 0.97 1. 0 0. 2 0.1 A 102 27 C 158 00 42 0.10 7.52 1.10 1.10 0.12 0. 21 0 aA 200 28 B 190 29 43 0. 04 9.8 0. 91 0.75 0. 2 0.1 A 188 31 o 148 75 1Water quenched from austenitiniug temperature. 2 Oil quenched fromaustenitizing temperature.

The general eifects of aluminum, molybdenum, manganese, silicon andchromium on yield strength and toughness are reflected in Table IV foralloys Nos. 21 through 34 which were treated in the manner described inconnection with Tables I and II.

As indicated above herein, in providing steels characterized by maximumtoughness it is most advantageous that the levels of phosphorus andsulfur not exceed about 0.01%. This is illustrated by the data of TableV. In this connection, 8 plate specimens each /2 inch thick were stackedin a suitable fixture and water quenched. This was done to simulate theexpected cooling rate of a steel plate 4 inches in thickness. pared byvacuum induction melting (30 lb. melts) followed by forging and hotrolling to plate, the finishing temperature being about 1800 F. Thesteels were austenitized at 1600 F. (held 1 hour), water quenched andindividually tempaged at 1050 F. for about 2 hours and then cooled. TheCharpy V-Notch impact values The steels were pre- The results set forthin Table VI reflect that a high level of toughness can be achieved byoveraging. Of course, as will be understood by those skilled in the art,the overaging temperature cannot be selected indiscriminately if theattributes of overaging are to be obtained. This is purposely manifestedby a comparison of Alloys Nos. 42 and 41 which have similarcompositions. Alloy No. 42 aged at 1050 F. did not exhibit much by wayof increased toughness and the strength level remained about the same.This indicates that at 1050" F. Alloy No. 42 was at about its peak agingtemperature, i.e., at 1050 F. little overaging occurred. Alloy No. 41afforded a tremendously higher level of toughness when aged at 1100 F.and the yield strength was still over 150,000 p.s.i. However, if abetter balance of strength and toughness would be necessary for suchsteels, it can be obtained with an intermediate overaging temperature of1075 F. Thus, as referred to herein, overaging affords a simpleexpedient for achieving desired combination of properties withoutsacrifice of hardenability and is particularly beneficial in providingsteel .plate of substantial thickness without loss of through hardening.

If, upon austenitizing, liquid quenching cannot be em ployed,air-cooling techniques can be utilized as noted hereinbefore,particularly with respect to steels containing at least 7% nickel, e.g.,7.5% to 10% nickel, and yield strengths of the order of 150,000 p.s.i.and higher can still be attained. This is illustrated by the data inTable VII which pertains to steels which were air-cooled from anaustenitizing temperature of 1600 F. and then aged at 1000 F. for 2hours.

ganese in an amount up to 0.5%, silicon in an amount up to 0.5%, up to5% cobalt, up to 2% copper and the balance essentially iron.

2. As a new article of manufacture, a steel plate made from the alloysteel set forth in claim 1.

3. The alloy steel as set forth in claim 1 wherein the carbon is presentin an amount not exceeding 0.2%.

1 Bar, 0.75 inch diameter. 2 Plate, 0.5 inch thick.

It is perhaps worthy of mention, that when the nickel content is betweenabout 7% and up to about 7.5% and air cooling is employed, it isadvantageous that the carbon content be at least 0.08%, e.g., 0.1% to0.15% carbon. Carbon does contribute to hardenability and strength in amore pronounced manner for the air-cooled steels as compared with quenchsteels. It should also be noted that air cooling a 0.5-inch thick plateis approximately equivalent to water quenching a plate 6 inches inthickness. This aspect coupled with the data in Table VII serves toemphasize that plates of substantial thickness can be provided with ahigh degree of hardenability.

In view of the excellent combination of mechanical propertiescharacteristic of the low alloy tempaging steels Within the invention,the steels are suitable for wide commerical application includingstructural members, e.g., girders and beams, pressure vessels, highstrength forgings including aircraft undercarriage forgings, fluidstorage containers, armor plate, etc., as well as for dies and nitridedparts, e.g. gears, etc.

As will be readily understood by those skilled in the art, the termbalance or balance essentially when used to refer to the iron content ofthe steels does not exclude the presence of other elements commonlypresent as incidental elements, e.g., deoxidizing and cleansingelements, and impurities ordinarily associated therewith in amountswhich do not adversely affect the basic characteristics of the steels.Up to 5% cobalt and up to 1.5% or 2% copper can be present in the steelsof the invention. Tungsten can be used to replace the molybdenum inwhole or in part; however, molybdenum is significantly more elfectivethan tungsten and it is thus preferred to use the former. Carbideformers such as vanadium, columbium, and titanium are quite unnecessaryand, more importantly, the use of these elements has been found to becausative of detrimental embrittling effects.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. lSuch modifications and variations are considered to beWithin the purview and scope of the invention and appended claims.

We claim:

1. A novel, low alloy tempaging steel characterized in the agedcondition by a yield strength of up to about 200,000 p.s.i., a Rockwellhardness of at least about Rc 37 and up to at least Re 43 and a CharpyV-Notch impact energy level of about at least 20 ft.-lbs. and having acomposition within the following ranges: 0.04% and up to not more than0.25% carbon, at least 4.5% and up to 10% nickel, at least 0.6% and upto about 1.2% aluminum, the ratio of nickel to aluminum being at least 5to 1, about 0.5% to 2% molybdenum, up to 1.5 chromium, man- 4. The alloysteel as set forth in claim 3 wherein the nickel is present in an amountof at least 5% and the aluminum is present in an amount of at least0.7%.

5. The alloy steel as set forth in claim 1 in which the carbon is from0.04% to about 0.16%, the nickel is from about 5% to 10%, the aluminumis from about 0.7% and up to not more than about 1.2%, the ratio ofnickel to aluminum being at least 5 to 1, the molybdenum is from about0.75% to about 1.5%, the chromium is up to 1%, the manganese is up to0.3%, and the silicon is up to 0.3%.

6. As a new article of manufacture, a steel plate made from the alloysteel set forth in claim 5.

7. The alloy steel as set forth in claim 5 wherein the nickel is presentin an amount of at least and the aluminum content does not exceed about1.1%.

8. The alloy steel as set forth in claim 7 wherein the carbon is presentin an amount not exceeding about 0.1%.

9. The alloy steel as set forth in claim 5 in which the nickel is fromabout 7% to 10%, the aluminum is from about 0.8% to about 1.1% and themolybdenum is from about 0.8% to about 1.5%.

10. The alloy steel as set forth in claim 9 wherein the nickel ispresent in an amount of at least 7.5%.

11. The all-0y steel as set forth in claim 9 wherein the carbon ispresent in an amount not exceeding about 0.1%.

12. Anew and improved process for heat treating nickel-aluminumtempaging steels to obtain yield strengths of about 150,000 p.s.i. to200,000 p.s.i. together with Charpy V-Notch impact energy levels of atleast 20 ft.-lbs. and up to at least 50 ft.-lbs. and Rockwell hardnessesof at least Re 37 and up to at least Rc 43 which comprises heating a lowalloy tempaging steel containing 0.04% and up to not more than 0.25carbon, at least 4.5 and up to 10% nickel, at least 0.6% and up to about1.2% aluminum, the ratio of nickel to aluminum being at least 5 to 1,about 0.5% to 2% molybdenum, up to 1.5% chromium, manganese in an amountup to 0.5%, silicon in an amount up to 0.5%, up to 5% cobalt, up to 2%copper and the balance essentially iron to a temperature of about 1550F. to about 1700 F., cooling said alloy steel, reheating said alloysteel to a temperature of about 950 F. to 1125 F. and thereafter coolingsaid steel.

13. A novel, low alloy tempaging steel having a composition within thefollowing ranges: 0.04% and up to about 0.2% carbon, at least 4.5 and upto 10% nickel, at least 0.6% and up to about 1.1% aluminum, the ratio ofnickel to aluminum being at least 5 to 1, about 0.5 to 2% molybdenum, upto 1.5% chromium, manganese in an amount up to 0.5%, silicon in anamount up to 0.5%, and the balance essentially iron.

No references cited.

DAVID L. RECK, Primary Examiner. C. N. LOVELL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,284,191 November 8, 1966 Peter Paul Hydrean et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 3, line 6, for "inergy" read energy lines 14 and 15, for"aluminun" read aluminum column 4, line 35, for "austentite" readaustenite lines 61 and 62, for "produccing" read producing column 5,line 12, for "conductive" read conducive columns 7 and 8, strike out"TABLE IV", in its entirety, and insert the same table after "Tables 1and II." in line 61, column 9; column 11, line 26, for "quench" readquenched lines 34 and 35, for "commerical" read commercial line 69, for"about" read above Signed and sealed this 19th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A NOVEL, LOW ALLOY TEMPAGING STEEL CHARACTERIZED IN THE AGEDCONDITION BY A YIELD STRENGTH OF UP TO ABOUT 200,000 P.S.I., A ROCKWELLHARDNESS OF AT LEAST ABOUT RC 37 AND UP TO AT LEAST RC 43 AND A CHARPYV-NOTCH IMPACT ENERGY LEVEL OF ABOUT AT LEAST 20 FT.-LBS. AND HAVING ACOMPOSITION WITHIN THE FOLLOWING RANGES: 0.04% AND UP TO NOT MORE THAN0.25% CARBON, AT LEAST 4.5% AND UP TO 10% NICKEL, AT LEAST 0.6% AND UPABOUT 1.2% ALUMINUM, THE RATIO OF NICKEL TO ALUMINUM BEING AT LEAST 5 TO1, ABOUT 0.5%TO 2% MOLYBDENUM, UP TO 1.5% CHROMINUM, MANGANSES IN ANAMOUNT UP TO 1.5% CHROMIUN, MANTO 0.5%, UP TO 5% COBALT, UP TO 2% COPPERAND THE BALANCE ESSENTIALLY IRON.